The construction of large underground tunnels is usually accomplished by removal of material at the tunnel face by machine (or otherwise) and subsequently lining the exposed tunnel surface with prefabricated liner members as the tunnel progresses through the ground. Usually, the tunnelling process will be assisted or in some instances made possible by the use of compressed air in the tunnel during the construction of the tunnel, as is usual in a plenum tunnelling process.
The above process has been in use since late in the nineteenth century and has been used successfully to drive tunnels through wide varieties of soil. If the soil through which the tunnel is being driven is impervious, the tunnel driving process is quite straightforward, and a minimum of air pressure will be required to allow the tunnel process to progress.
If, however, the tunnel must be driven through areas where pockets or strata of pervious material are present in the plenum area, problems with the ingress of sand and water into the plenum area act to impede the tunnelling process.
Pervious materials can cause problems for either of two reasons. In cases where the pervious material allows the air to flow directly to the atmosphere the pressure in the tunnel continuously escapes because the air leaks out, hitherto the only remedy has been to increase the air supply to the tunnel until the supply belances the leakage at the desired pressure. In cases where the pervious materials are totally enclosed by impervious materials, the pervious formation can fill up with air so that the air acts on the water within for formation and allows it to flow into the tunnel, no good solution has been found for this second problem in shield driven tunnels. Tunnellers have had to rely on various methods of face breasting with planks, straw, and other materials to restrain the flow.